Single crystal electroluminescence



July 1, 1958 w. w. PIPER 2,841,730

SINGLE CRYSTAL ELECTROLUMINESCENCE Filed Dec. 5, 1955 I I I M m 4 1 I I i /nvem0r:

United States Patent 2,841,730 SINGLE CRYSTALELECTROLUMINESCENCE William W. Piper, Glenville, N. Y., a'ssignor to General Electric Company, a corporation of New York Application December 5 1955,,Serial No. 550,992

9 Claims. (Cl. 313-408) The present invention relates to electroluminescent cells, and, more particularly, to electroluminescent cells utilizing a single crystal of an activated electroluminescent phosphor as the light emitting member. This application is a continuation-in-part of my copending application Serial No. 274,237, filed February 29, 1952, now, abandoned, and assigned to the same assignee as the present invention.

Electroluminescent cells, or luminous capacitors, as they are sometimes called, resemble a fiat plate capacitor wherein the dielectric has incorporated within it a quantity of electroluminescent phosphor and one of the plates is a transparent conducting layer. When an alternating current potential is impressed between the two capacitor plates luminosity effects are observed within the dielectric layer. A device of this character is disclosed and claimed in the application of Jerome S. Prener, Serial No. 245,696, filed September 8, 1951, now Patent 2,731,423, and assigned to the same assignee as the present invention. I j i e The electroluminescent cells of the prior art operate only on alternating current, and the intensity of light given off thereby increases with voltage and frequency up to a certain point. Beyond a certain point, higher frequency is not accompanied byincreasing brightness. The limiting factor on raising the voltageis dielectric brealc down. a

It is an object of this invention to provide an electroluminescent cell which will operate upon unidirectional as well as alternating current potentials.

Another object of the invention is to provide a light producing unit utilizing-a single crystal of an electroluminescent phosphor.

A further object of the invention is to provide an electroluminescent cell of small dimensions utilizing a single crystal of phosphor as the light emitter.

A further object of the invention is to provide a method for producing electroluminescent cells composed of the single crystal of an electroluminescent phosphor.

Briefly stated, in accordance with one aspect of my invention, I provide an electroluminescent cell by producing a single crystal containing a metal activator and one or more substances selected from the group consisting of zinc sulfide, zinc selenide, cadmium sulfide, cadmium selenide, and positioning a pair of oppositely disposed electrodes on the crystal.

The novel features belived characteristicof the invention are set forth in the appended claims. The invention itself, however, together with further objects and advantages thereof may best be understood by referring to the following description taken in conjunction with the drawing in which Figures 1 and 2 show, in greatly enlarged form, electroluminescent cells constructed in accord with the invention.

In Figure 1 of the drawing, an electroluminescent cell constructed in accord with the invention comprises a single crystal 10 of an electroluminescent phosphor having a pair of oppositely disposed transparent electrodes 11 and 12 in goodelectrical contact with opposite surfaces 2,841,730 Patented July 1, 1958 ICC thereof. For the purposes of this application and the appended claims, a single crystal of an electroluminescent phosphor material may be defined as one having the following characteristics. A single crystal of a phosphor material generally has a high degree of crystal perfection and is in the form of a regular elongated column having a cross-sectional area which is polygonal in shape. One major dimension of the single crystal cross-sectional area is at least one millimeter in length. The length of the column comprising the single crystal is generally much greater than the cross-sectional dimension thereof, being at least one millimeter, and in many cases being as large as one centimeter. The minimum weight of single crystals of phosphor materials utilized in this invention is approximately one-half milligram, although some large single crystals may weigh as much as one gram. Such regular geometry and dimensions, and high degree of crystal perfection are to be contrasted with the microcrystals comprising conventional powdered phosphors utilized in electroluminescent cells heretofore. Such microcrystals may have any irregular geometry, may be fused masses which include a plurality of unoriented crystals and may have dimensions from one to forty microns. The single crystals of this invention are also to be distinguished from large crystalline masses of phosphor materials grown by conventional rapid evaporation and condensation techniques in which case the crystalline phosphor body rather than being a single crystal comprises a plurality of small microcrystals with no particular orientation and a lesser degree of crystal perfection.

The single crystal phosphor materials of this invention may also be defined as crystals which are grown by a slow sublimation crystal growth process upon a heated base plate which is maintained at a temperature slightly below the condensation point of the material comprising the crystal. Such a method of crystal formation is described in greater detail hereinafter.

Referring again to Figure 1 of the drawing, electrodes 11 and 12 may be composed of an opaque conducting substance such as silver paste or a smooth surfaced metal plate or metallic probes. It may be desirable in certain cases to have one or both of electrodes 11 and 12 composed of a transparent conducting layer, such as a glass backed layer of conducting tin oxide or titanium dioxide. if both electrodes are opaque, luminosity may be readily detected emanating from the exposed portions of the crystal 10 when electrodes 11 and 12 are connected to an energizing potential source.

A source of electrical energy 13 is connected to the electrodes 11 and 12 by means of conductors 14 and 15 respectively. Source 13 may be either a source of unidirectional or alternating potential, since the single crystal electroluminescent cells of this invention,'as opposed to the polycrystalline electroluminescent cells of the prior art which luminesce only in response to alternating current potentials, become luminescent when excited by either alternating or unidirectional potentials. A suitable exiting potential may be any source sutfici ent to impress an electric field of approximately 10 volts per centimeter across single crystal 10.

Crystal 10 may be prepared from a charge comprising one or more materials selected from the group consisting of zinc sulfide, zinc selenide, cadmium sulfide and cadmium selenide. These materials are mixed with a small quantity of a metallic activator generally in the form of a chloride since it has been found that the chloride ion enhanced the activation attained thereby. The mixture is then placed in finely divided form in a carefully cleaned quartz tube. The tube is then evacuated and heated to drive off absorbed gases, and sealed off. The lower half of the tube containing the finely divided mixture is then heated to a temperature at which appreciable sublimation 0.067% by weight of copper.

of the charge occurs hereinafter referred to as the sublimation temperature of the charge. If the charge comprises zinc sulfide this temperature is approximately 1175 C. For zinc selenide or cadmium sulfide the temperature is approximately 800 C. For cadmium selenide the temperature is approximately 600 C. The top half of the tube is maintained at a temperature approximately 100 C. cooler so that this portion of the tube is below the condensation temperature of the vapor sublimed from the charge. The powder then gradually sublimes from the bottom of the tube and condenses onto the cooler walls of the upper portion of the tube in the form of large single crystals.

lreferred activating agents from my phosphors are copper and silver, but other activators such as manganese and phosphorus and other well known electroluminescent activators are also satisfactory. The activating agents used in. the practice of my invention are added to the host phosphor material placed in the evaporated quartz tube in proportions which may range from approximately 0.01% to 5% by weight of the host phosphor. The addition of quantities of activator less than approximately 0.01% by weight does not produce high brightness electroluminescent crystals. The addition of amounts of activator, particularly highly conductive activators such as copper and silver in excess of 5% by weight tends to decrease the electrical resistivity of the resultant single crystals and lowers the dielectric breakdown voltage thereof. A preferred range of activator percentages which result in the formation of crystals having excellent light producing characteristics is from approximately 0.05 to 1% by weight of the host material. The activator does not need to be initially present in elemental form, and is preferably added as the chloride. If copper is to be the activator incorporated in the host phosphor material, it may be added in the form of copper chloride to the powdered phosphor material placed Within the quartz tube.

it is to be noted that the above quoted percentages are not necessarily the amount of activator present in the resultant single crystal, but represent the amount of activator added to the host phosphor material before the sublimation crystal growth of single crystals. I have found experimentally that as the percentage of the metallic activator added to the host phosphor in the initial charge is increased the percentage of the activator to be found in the final crystal decreases. Thus, for example, when 0.01% by weight of copper is added to a zinc sulfide charge substantially the same percentage of copper is found in Zinc sulfide single crystals grown in accord with this invention. On the other hand, when 0.2% by weight of copper is added to a zinc sulfide charge and single crystals grown therefrom in accord with this invention the resultant crystals are found to contain only approximately Additionally, single crystals of zinc sulfide grown from a charge initially containing 1% by weight of copper are believed to contain at most 0.2% by weight of copper.

The method by which I produce single crystals of phosphor materials may be utilized to co-crystallize two or more substances. For example, an excellent phosphor is produced from an initial mix of eighty parts zinc sulfide to 20 parts zinc selenide and 0.5% by weight of copper as the chloride.

it is desirable to allow crystal formation to take place over a protracted period of time under conditions of slow crystal growth. Forty-eight hours is about the minimum growth time to obtain satisfactory crystals. Obviously, this time may be shortened, but the crystals produced will not be large enough to give satisfactory results.

In one specific example of the formation of single crystals, ten grams of zinc sulfide and 22 milligrams of copper chloride were deposited in the bottom of a closed quartz tube 6" in length and 1" in diameter. The tube was evacuated and heated to a temperature approximately seamen 250 C. to drive ofi absorbed gases, and sealed 011. The lower half of the tube containing the charge was then heated to a temperature of approximately 1100 C. The upper half of the tube was maintained at a temperature of approximately 1000 C. These temperatures were maintained for approximately 60 hours. At the end of the 60 hour period the tube was opened and a number of long needle-like single crystals of zinc sulfide activated with copper were found to have grown on the walls of the low temperature portion of the tube.

A single crystal produced by the above-described method is then removed from the sealed tube and two oppositely disposed surfaces are carefully coated or otherwise contacted with conducting electrodes. This may be accomplished by vapor depositing a thin transparent metallic film upon oppositely disposed sides to form electrodes 11 and 12. Alternatively, electrodes 11 and 12 may comprise transparent conducting coatings of tin oxide or titanium dioxide deposited upon glass backing plates and maintained inpressure contact with oppositely disposed surfaces of the single crystal. The unit thus produced makes a satisfactory electroluminescent cell which operates either on alternating current or unidirectional current potential. A number of these small crystals may be combined into a single electroluminescent lamp interposed in parallel between the same electrodes if desired. This may be accomplished by orienting a plurality of single crystals into a mosaic screen and contacting parallel sides of the properly oriented crystals with oppositely disposed electrodes, at least one of which shouldbe transparent.

The operating characteristics of the electroluminescent cells of the invention are dependent upon the orientation of the electric field with respect to the unique or C-axis of the crystal. The brightness for a given field is much higher when the field is oriented perpendicular to the unique axis of the crystal as is shown in Figure 2. Properly oriented crystals have been observed to emit six times the brightness of non-oriented crystals.

While the present invention has been described with reference to particular embodiments thereof it will be understood that numerous modifications may be made by those skilled in the art without departing from the invention. It is my intention therefore by the appended claims to cover all such equivalent variations as come within the true spirit and scope of the foregoing disclosure.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. An electroluminescent cell comprising a single crystal consisting essentially of an activated phosphor material selected from the group consisting of zinc sulfide, zinc selenide, cadmium sulfide, cadmium selenideand mixtures thereof, and a pair of electrodes on oppositely disposed surfaces of said crystal.

2. An electroluminescent cell comprising a single crystal consisting essentially of an activated phosphor material selected from the group consisting of zinc sulfide, zinc selenide, cadmium sulfide, cadmium selenide and mixtures thereof, said material being intermixed with from 0.01 to 5% by Weight of an electroluminescent activator, and a pair of electrodes on oppositely disposed surfaces of said crystal.

3. An electroluminescent cell comprising a single cryselectrodes on oppositely disposed surfaces of said crystal.

6. An electroluminescent cell comprising a single crystal consisting essentially of zinc sulfide co-crystallized with zinc selenide and from about 0.01 to 1.0% by weight of an activator selected from the group consisting of copper and silver and a pair of electrodes on oppositely disposed surfaces of said crystal.

7. An electroluminescent cell comprising a single crystal consisting essentially of zinc sulfide co-crystallized with zinc selenide and a copper activator in the approximate proportion 80 parts by weight of Zinc sulfide to 20 parts by weight of zinc selenide and. 0.5% by weight of copper.

8. An electroluminescent cell comprising a single crystal consisting essentially of a material selected from the group consisting of zinc sulfide, zinc cadmium sulfide, Zinc selenide, cadmium selenide and mixtures thereof activated with approximately 0.01 to 1% by weight of a material selected from a group consisting of copper, silver, manganese and phosphorus, and a pair of conducting electrodes positioned on oppositely disposed surfaces of said crystal parallel to the unique axis of the crystal.

9. The method of preparing a single electroluminescent crystal which comprises placing a mixture of at least finely divided zinc sulfide and from 0.01 to 5% by weight of a metal activator in a tube, evacuating and sealing said tube, heating the portion of said tube with which the mixture is in contact approximately to the sublimation temperature of the zinc sulfide mixture, maintaining at least a portion of the remaining surface of said tube at a slightly lower temperature to provide a surface for the slow condensation of volatile particles, removing the condensed crystal from the tube and contacting two oppositely disposed surfaccs of said crystal with conducting electrodes at least one of which is transparent.

References Cited in the file of this patent UNITED STATES PATENTS 2,462,517 Leverenz Feb. 22, 1949 2,566,349 Mager Sept. 4, 1951 2,600,579 Ruedy et a1 June 17, 1952 2,624,857 Mager Jan. 6, 1953 2,684,450 Mager et al July 20, 1954 2,721,950 Piper et a1. Oct. 25, 1955 

1. AN ELECTROLUMINESCENT CELL COMPRISING A SINGLE CRYSTAL CONSISTING ESSENTIALLY OF AN ACTIVATED PHOSPHOR MATERIAL SELECTED FROM THE GROUP CONSISTING OF ZINC SULFIDE, ZINC SELENIDE, CADMIUM SULFIDE,S CADMIUM SELENIDE AND MIXTURES THEREOF, AND A PAIR OF ELECTRODES ON OPPOSITELY DISPOSED SURFACES OF SAID CRYSTAL. 